Electrical translation circuits



' 28, 3341., R J, LARSEN ELECTRICAL TRANSLATEQN CIRCUITS 3 Sheeis-Sheet 1 Filed Jan. 3, 1958 INVENTR. *3.30111 P. J. LARSEN 'ELECTRICAL TRANSLATION CIRCUITS Jan. 28, li'l.

. Filed Jan. 5, 1938 s sheetsfsheet 2 2 0 lill llllllk 17 a G-il 1N VEA/TUR Paul oar-sen ATTUKM" AMPL PIER DETECTORHAMPLIFIER AMPLI FXER Fl LTER 05C! LLATURI 107 i911/ Jan., 28, il. Pg J. ARSEN mcTRIcAL TRANSLATION' CIRCUITS g'iied Jam :5, i938 s sheets-sheet 3 Patented Jan.

aimeraient,

SLTKN GUTES Paul i. Larsen, East Grange, N. li., assigner to Radio Patents Corparation, New York, N., Y., a corporation of New York Application January 3, 1938, Serial No. 183,15d

13 Claims.

The present invention relates to improvements in and a method of amplifying variable electric currents as used in the high frequency and low frequency arts such as in radio, telephone, telegraph, talking picture, sound systems and the like. More particularly, the invention is concerned with amplifiers embodying one or more inverse feedback arrangements for stabilizing the operation, preventing distortion and to obtain other advantages Well known.

The object of the present invention is to cbtain a frequency discrimination by utilizing the effect of inverse feedback in an amplifier, more particularly to provide an amplifier with a more or less square top or band-pass or band suppression frequency characteristic in a simple manner and substantially without the use of resonating elements or circuits with their inherent drawbacks such as due to mutual coupling effects both internal and external requiring careful screening and/or shielding and which are liable to produce distortion, instability conditions and other defects well known.

Another object is the provision of an ampliner having a predetermined frequency response characteristic or band width suitable for amplifying and/or translating both low frequency (audio), intermediate frequency and high signal frequency bands of any desired frequency range or band width and comprising coupling and transmission networks consisting substantially of resistance and capacity elements.

A further Objectis to combine capacity and resistance elements as coupling networks in an amplifier circuit-in such a manner as to obtain a desired frequency band amplification or transmission above or below a first predetermined cutoii frequency and to balance or neutralize by inverse feedback all the currents having frequencies above or below another predetermined frequency thereby to obtainV a frequency response characteristic of the band-pass or band suppresf sion type positioned at any desired point on the frequency scale extending from zero frequency to the low (audio), intermediate and high frequency ranges employed in practice.

These and further objects-and advantages of the invention will become more apparent as the following detailed description proceeds .taken with reference to the accompanying drawings illustrating several embodiments of the invention and wherein 4 Figure 1 shows a simple amplifying circuit having a band-passA type frequency discriminating amplifying or transmission characteristic constructed in accordance with the invention, Figure 2 shows frequency'response curves illustrative of the function and'operation of the circuit-according to Figure l, Figure 3 shows a modification of an amplifying circuit with band-pass characteristics of the type shown by Figure 1, Figure d is a further modification of a circuit according to the invention, Figure 5 illustrates another modification of an amplifying circuit constructed in accordance with the principle of the invention, Figure 6v shows frequency response curves explanatory of the function of thel circuit shown in Figure 5, Figures 7 and 8 illustrate amplifying circuits according to a still further improved embodiment oi' the invention adapted to increase the sharpness or cut-on effect of the band-pass characteristic, Figure 9 is a diagram illustrating the function of the circuits according to Figures 7 and 8,' and Figures l0 and 1l are block diagrams illustrating the employment of the invention in the intermediate section of a superheterodyne radio receiver or the like. Similar reference characters identify simila parts throughout the dierent views of the drawings.

Referring to Figure l wherein is shown a simple inverse feedback amplifying circuit having a predetermined frequency discriminating characteristic of the band-pass type positioned at any desired point ofthe frequency scale including the audio, intermediate and high `frequency ranges, numeral i@ represents an amplifying valve of standard construction, in the example shown a valve ofthe pentode type, having a cathode li, an input or control grid l2, a screen grid i3, a suppressor grid it and a plate or anode i5. The suppressor grid ld is directly internally connected with the cathode in a manner well known.- Item i6 represents a network comprising a plurality of capacity units connectedv in series and resistance elements in shunt. The input of network i6 is connected to terminals a, b forming the input. for the amplier through a coupling condenser I'i which may form part'of the network I6 in combination with shunt resistance 26. connectedacross the grid cathode path of the valve I0. As is known, networks of this type are effective in preferredly passing currents of higher frequency abovea predetermined cut-off frequency and of suppressing currents of lower frequency below said cut-off frequency, by reason of the fact that the high frequency currents are easily directly transmitted through the capacity The output of the network IB` is units while the low frequency currents are shimted or by-passed through the resistance units of the system. Arrangements of this type with capacity units in series and shunted by resistance v elements are also known as high-pass resistance capacity filters or networks.

In this manner, current variations beyond a predetermined frequency known' as the cut-oi! frequency of the network or filter I5 impressed upon the input terminals a, b which maybe of low frequency (audio), intermediate or high frequency are transmitted and amplified in the out. put circuit of the valve I0. This is further illustrated in Figure 2a of the drawings, wherein A represents the response characteristic-output current or signal response in suitable units such as in decibel units as shown vs. frequency-of the high-pass filter or network I5 having a cutoiI frequency f1. I

The amplified current variations produced in the output or plate circuit of valve I may be impressed in the known manner through a load resistance I8 and coupling capacity I9 to a further amplifying or utilization circuit to be connected to the outputterminals c, d of the system. Numeral 20 represents the high tension source for supplying anode current to the valve III. The screen grid I3 is shown connected in a known manner to a suitable tap point of the high tension source 20 through a voltage drop resistance 2I and is by-passed to cathode or ground by a capacity 22 in a known manner. There is further provided a biasing resistance 23 inserted in the cathode return lead of the valve shunted by a by-pass condenser 24 to provide suitable grid biasing potential and to cause the valve I0 to operate at a favorable point of itsv operating characteristic.

'I'here is provided in accordance with the improvements of the invention a further resistance capacity type high-pass filter or network 25 in shunt to the anode cathodepath of the valve Ill on the one hand and having its. output connected to a variable tap point of the first or input shunt resistance y26 of the filter or network I in the grid circuit. 'I'he filter or network 25 is designed to have a characteristic such as' shown at B in Figure 2a with a cut-oi! frequency fr differing from the cut-off frequency f1 of the network IB by a predetermined frequency range equal to the desired band width or transmission characteristic of the system.. Since, as is Well' known, the anode potential in an amplifying valve of the type disclosed varies inversely to the controlling potential variations impressed upon the grid of the valve, it is seen that the potential impressed upon the grid circuit through the filter or network 25 is substantially 180 out of phase with the potential impressed upon the grid from an input circuit through the network I5. Both the input control potential and the inverse feedback potential are thus superimposed in parallel and impressed upon the grid of the valve. Since the network 25 has a characteristic as shown at B in Figure 2a with a higher cut-off frequency f2 than the network I5, it is readily seen that the inverse feedback as described will have the effect of suppressing or neutralizing all frequencies above fr in such a manner that a resultant band-pass type response characteristic is obtained having a width b equal to the frequency range between f1 and fz as shown at C in Figure 2b.

The filters or networks I 6 and 25 may be designed to have the requisite critical or cut-Oil' frequencies f1, fz in a known manner by proper dimensioning of the capacity and resistance units in relation to each other and to the impedance of the associate circuits connected to the input output terminals of the networks or filters.

The coupling resistance I8 in the plate circuit of the valve I0 may be replaced by an inductance which latter has the advantage of obtaining increased controlling potential applied to the next amplifying stage or utilization circuit while in no way interfering with the function and frequency discriminating action of the inverse feedback system. In this manner, the inherent loss in gain or amplication of thesystem dueto the effect of the inverse feedback may be compensated to a certain extent. l

Referring to Figure 3, there is shown a similar system to that illustrated in Figure 1. According to this modication, a multi-stage amplifier 42 indicated lschematically is provided with the input of the network 25 in the inverse feedback circuit connected across a variable portion of an anode load resistance I8 in the plate circuit of the last valve-in which case an odd number of amplifying valves must be employed-on the one hand, and with the output of the network 25 connected in the cathode lead from the input lter or network I6 on the other hand. The arrangement according to Figure 3 further differs from Figure 1 in that the output from the filter 25 is impressed serially with the network I5. j

While in the previous figures high-pass filters or networks are provided in both the input and feedback circuit paths, it is understood that substantially the same effect will be obtained by using low-pass illters of the resistance capacity type in the input and 'feedback circuits. Filters or networks of the latter type comprise a number of resistance elements'ln series and capacity elements in shunt and are effective in passing the frequencies below and suppressing frequencies above their cut-oil' frequency. In this manner, by properly choosing the cut-oil' frequencies of the networks in the input and inverse feedback circuits, respectively, a resultant band-pass characteristic or response curve as shown at C in Figure 2a may be obtained in substantially a similar manner as described herelnbefore.

An arrangement of this type is shown in Figure 4. In the latter, a low-pass resistance capacity network 45 is connected between the input terminals a. b and the grid cathode path of the valve I0 through further coupling capacities I1 and 46, respectively, and grid leak resistance 41. Similarly, a low pass resistance capacity network 48 is inserted in the negative feedback path in series with a coupling capacity 49 connected to the anode I5 on the one hand, and having an output terminal connected to a variable tap point of the input resistance 26 connected across the network 45. If the latter has a characteristic cut-olf at frequency f2 and the network 45 has a characteristic cut-oil. at frequency f1 a resultant inputoutput band-pass characteristic will be obtained as shown at C in Figure 2b. There is furthermore shown a coupling inductance coil 35 in place of a resistance in the plate circuit for the purpose and with the advantage as pointed out hereinabove.

As previously pointed out, in inverse feedback circuits the amplification or gain is reduced due to the negative feedback so that more amplifying stages are required to produce output potentials or currents of desiredstrength compared with ordinary amplifiers without inverse or negative 'resistance 1 I.

feedback arrangements. In accordance with a further improvement of the invention, this loss or decrease in gain may be compensated by an additional direct feedback providing regeneration of the limited band characteristic obtained.

Instead of inversely feeding back'energy from separate valves to the input circuit of a single preceding valve through high-pass and low-pass networks, respectively, to obtain a band-pass characteristic the same results may be obtained by employing two valves connected in parallel with common input and output circuits and by inverse feedback circuit paths from the anodes of both valves to the common input circuit through'high and low pass networks, respectively. An arrangement of this type is shown in Figure 5. In the latter, the valves I9 and 21 are connected to a common input circuit comprising coupling capacity I1 and input impedance 26. The anodes are connected in parallel through plate resistance or impedance I8 to -the plate supply and to a common output circuit through a coupling capacity I9. A rst inverse feedback circuit comprising a low-pass resistance capacity network 54 having a characteristic E according to Figure 6a is connected between the anode of valve I0 and a variable tap point of the input resistance 26,-while a further inverse feedback circuit including a high-pass resistance capacity network 55 having a characteristic as shown at F in Figure 6a. is connectedl between the anode of valve 21 and another tap point of the input resistance 26. The common input circuit I1 and 26 should have a substantially constant orv broad frequency response for the frequency band desired whereby through the effect of the inverse feedback circuits a resultant band-pass response characteristic is obtained as shown at G in Figure 6b. The valve 21 has a cathode 29, control grid 38, screen grid 3I, suppressor 32, and a plate 33. The screen grid 3l lis connectedto the high potential terminal of the plate potential source 20 through a resistance 38 by-passed by a condenser 39.

Referring to Figure 7 there is shown a system comprising a multiple section amplifying valve having high and low-pass resistance capacity networks in the input circuits thereof cooperating with low and high-pass networks in the inverse feedback circuits having cut-olf characteristics different from the others by a frequency difference of the band desired in such a manner that each amplier circuit provides the desired band-pass characteristic, however conjointly obtaining an improved resultant cut-off effect as well as increased efficiency and output signal strength.' There is shown in Figure 7 a composite amplifying valve 6U equivalent to two amplifying valves arranged in a common envelope and having a cathode 6I, a pair of control grids 82 and B3, and a pair of anodes 64 and 85, thus forming two separate and independent discharge or. amplifying paths.. The grid circuit of the amplifying path or section 5I, 62, 64 is connected to the input terminals a, vb through a low-pass resistance capacity network 66 by the aid of a coupling capacity 68 with associate coupling or grid-leak resistance 10. The grid or control circuit of the amplifying path or section SL53, 65

is similarly connected to input terminals a, b through a high-pass network and a coupling capacity 69 with associate grid-leak or coupling 'l2 represents a grid biasing resistance insertedV in the common cathode return lead shunted by by-pass capacity 13. The anodes 84 and 85 are connected in parallel to the common output terminal c through anode load resistances 14, 18 and associate coupling condensers 18, 11'. Item 18 represents-"a common high tension source for supplying the anode current. Both anodes 64 and 85 are further shunted to the cathode through low-pass and high-pass resistance capacity networks, respectively, shown at 18 and 80 and having their output terminals connected to suitable tap points of the respective coupling resistances 10 and 1I in the grid circuits. The operation of this system is illustrated in Figure 9. In the latter, H represents the response characteristic of the network 68, J represents the response characteristic of the network 61, both networks being designed to overlap with cut-off frequencies fz, f1, respectively, spaced by a predetermined frequency range equal to the desired band width b of the system. Furthermore, K represents the characteristic of the network 19 and L represents the characteristic of the network 80 in Figure 7.

As will be seen from Figure 9, the result of the combined action of all four networks in the inputl and inverse feedback circuits will be a curve as shown at M in Figure 9 representing a band-pass characteristic having a desired width b with improved cut-ofi effect and increased gain or output signal strength.

A similar combined effect of a plurality of low and high-pass networks in the input and inverse feedbackcircuits may be obtained by a circuit of the'type shown in Figure 8. The latter differs from Figure '7 in that a pair of low-pass and high-pass networks 82 and 83 having cut-ofi characteristics as shown at H and J in Figure 9 are connected in series across the input circuit of a single valve I0 arranged and connected as shown at previous figures. There are further shown a pair of inverse feedback circuits with low-pass and high-pass networks 85 and 84Y having their inputs connected across the anode cathode path of the valve and having a variable portion of their voutput potentials obtainedfrom resistances 86 and 81 serially inserted between one output terminal of the ntwork 83 and the cathode or negative potential point of the system. If the characteristic of the network 84 is similar to that shown at L and the characteristic of the network 85 equal to the characteristic as shown at K in Figure 9, it is understood that a combined resultant band-pass eect is obtained in a substantially similar manner as described hereinabove.

It is evident from the above that the novel amplifying circuit arrangement according to the invention may be used and embodied in various systems where it is desired to obtain a frequencyl intermediate frequency stages ofthe type constructed according to the invention.v

Referring to Figure l0, the receiver of conventional construction comprises an antenna |85, a

RF or high frequency signal amplifier |08 connected to a frequency converting device or mixer IIS wherein the high frequency signals are changed into intermediate frequency signals by combination with locallyl produced oscillations obtained from an oscillator |01. yThe intermediate frequency signals are passed through a resistance capacity filter |09 having definite cut-oil! characteristics and applied to an amplifier H0.

From the latter, a portion of the amplified energy is inversely fed back through a resistance capacity filter of specific construction and cut-off characteristics and applied to the filter |09 or input of the amplifier H0 in accordance with any one of the modifications or combinations thereof disclosed and described hereinbefore. After the intermediate frequency signals have been selectively amplified to-a desired degree, they are impressed in a known manner upon a rectifier or detector I2 for conversion into low or audio frequency signals which latter after further amplication .by an audio amplifier |13 serve to operate a suitable translating device such as a loud speaker shown at Ill. It is naturally obvious that in the audio frequency amplifier similar inverse feedback systems may be employed to give a desired band width of from about '10 to about 7000 cycles as an example thereby eliminating any possible cycle hum or hiss from the carrier or tubes from the loud speaker circuit.

Referring to Figure 11, there is shown a similar receiver of the superheterodyne type with an intermediate frequency section comprising two amplifiers H0 and H5 in parallel each amplifier having associated therewith an inverse feedback system including resistance capacity networks I I1 and III in the manner described hereinbefore.

In the embodiment of the invention illustrated, resistance capacity ty-pe networks have been shown to obtain high-pass and low-pass transmission characteristics.

It is understood that any other type of network or filter for producing the above effect such as low and high-pass filters comprising capacity and inductance elements may be employed with equal advantage and without departing from the spirit of the invention. It is further understood that the inventionqniay be equally employed for translating or amplifying both low frequency and high frequency signal waves such as audio or speech currents as well as intermediate frequency or high frequency waves or current. In the former case the coupling impedances, or if desired the inductance units when employing a resistance inductance type lter preferably consists of iron cored inductances or choke coils, while in the latter case inductance coils either without iron core or with cores of the known high frequency type may be employed.

While I have shown a particular combination of elements to illustrate one way of carrying out the invention, I want it clearly understood that any one of the elements can be used by itself or with any other element besides those shown and illustrated without departing from the scope of the invention.

It will further be evident from the above that the invention is not limited to the specific arrangements of parts and circuits shown and methods disclosed herein for illustration, but that the novel thought and underlying principle of the invention is susceptible of numerous variations and modifications coming within the broad scope and spirit of the invention as defined in the appended claims.

The specification and drawings are accordingly intended to be regarded in an illustrative rather than a limiting sense.

1. An electric wave band translation system comprising an input and an output and an amplifier connecting said input and output. the frequency response range of said system substantially exceeding on both sides the frequency range of the wave band to'be transmitted, a first stop filter network associated with said input circuit, and a feed-back path from said output to said input arranged to feed back amplified wave energy substantially in phase opposition to the incoming wave energy and including a second stop nlter network. said first network with its pass-range embracing said wave band and with its stop-range substantially covering said frequency response range outside one limit frequency of said wave band, said second network with its stop range embracing said wave band and with its passrange substantially covering said frequency response range outside the other limit frequency of said band range.

2. An electrichwave band translation system comprising an input and an output and an amplier connecting said input and output, the frequency response range of said system substantially exceeding on both sides the frequency range of the wave band to be transmitted, a first stop filter network associated with said input circuit, and a feed-back path from said output to said input arranged to feed back amplified wave energy substantially in phase opposition to the lncoming wave energy and including a second stop filter network, said first and second network being oi' the same type blocking substantially all frequencies on one side and passing substantially all frequencies on the other side of a predetermined frequency, said first network with its pass-range including said wave band and with its stop range substantially covering said frequency response range outside one limit frequency of said wave band, said second network with its stop range embracing said wave band and with its passrange substantially covering said frequency response range outside the other limit frequency of said wave band.

3. In an electric wave translation system, an amplifier having an input and an output and a main amplifying path connecting said input with said output, a plurality of circuit paths between points of higher and lower oscillation levels of said amplifier for inversely feeding back energy so as to reduce the gain of said amplifier, bandpass lters connected in both said main amplifying and feedback paths, part of said filters having an equal cut-ofi. frequency and the remaining filters also having equal cut-off frequencies differing from said first cut-off frequency by a predetermined amount, to obtain a resultant inputoutput band-pass characteristic of said system with improved cut-off effects and embracing the range between said first and second cut-off frequency, respectively.

4. An electric wave translation system comprising an input and an output and an amplifier connecting said input and output, the frequency response range of said system substantially exceeding on both sides the frequency range of the wave band to be transmitted, a path for feeding back amplified wave energy from said output to said input substantially in phase opposition to the incoming wave energy, a pair of highpass networks comprising a plurality of capacity elements in series each shunted by resistance elewave band to be transmitted, one of said networks being inserted in said main path of said amplil er and the other of said networks being arranged in said feedback path, the cut-on frequency of said nrst mentioned network substantially coinciding with the lower limit frequency and the cut-oir frequency of said latter network substantially coinciding with the upper limit frequency of said wave band.

5. An electric wave translation system comprising an input and an output and an amplier connecting said input and output, the frequency response range of said system substantiallyexceeding on both sides the frequency range of the wave band to be transmitted, a path for feeding back amplified wave energy from said output to said input substantially in phase opposition to the incoming wave energy, a pair of low-pass filter networks comprising a plurality of resistance elements in series each shunted by capacity elements and having cut-off frequencies differing from each other by a predetermined range substantially equalling the frequency range of the wave band to be transmitted, one of said networks being inserted in the main amplifying path between said input and output and the other network being arranged in said feedback path, the cut-off frequency ofsaid first mentioned network substantially coinciding with the upper limit frequency and the cut-olf frequency of'said latter network substantially coinciding with the lower vllixnit frequency of said wave band.

' 6. An electrical system for transmitting a wave band of desired frequency range comprising an electron valve amplifier having an input and an output, the frefiuency response range of said system including z nd substantially exceeding on both sides the frequency band to be transmitted, a. lowpass network having a predetermined cut-off frequency substai. tially coinciding with the upper limit frequency of said wave band connected to said input to produce amplified current variations in said output circuit below said cut-off frequency, a further-lowpass network having a cut-off frequency substantially coinciding with the lower limit frequency of said wave band, said further network being connected to said output, and connections from said further network to said input for feeding back amplied energy in inverse phase relation to the input energy and with such amplitude as to reduce the gain of said amplifier, to obtain a resultant band-pass type input-output frequency response characetristic embracing the range between said cut-olf frequencies and substantially suppressing frequencies outside said band to be transmitted.

'7. An electrical system for transmitting a wave band of desired frequency range comprising an electron valve amplifier having an input and an output, the frequency response range of said system substantially exceeding on both sides the frequency band to be transmitted, a high-pass network comprising a plurality of capacity elements in series each shunted by a resistance element and having a predetermined cut-o frequency substantially coinciding with the lower limit frequency of said wave band, said high-pass network being connected to said input to produce amplified current variations in said output above said cutoff frequency, a further high pass network comprising a plurality of capacity elements in series each shunted by a resistance element and having a cut-off frequency substantially coinciding with the upper limit frequency of said wave band, said further high pass network being connected to said output, and connections from said further network to said input for feeding back amplified energy in inverse phase relation to the input energy and with such amplitude as to reduce the gain of said amplifier, to obtain a resultant band-pass type input-output `frequency response characteristic embracing the range betweenl said cut-off frequencies and substantially suppressing frequencies outside said wave band to be transmitted.

8. An electrical system for transmitting a wave band vof desired frequency range comprising an electron valve amplier having an input and an output, the frequency response range of said sysof resistance elements in series shunted by ca- 1 pacity elements and having a cut-off frequency substantially coinciding with the lower limit frequency of said w-ave band, said further low-pass network being connected to said output, and connections from said further network to said input circuit for feeding back amplified energy in inverse phase relation to the input energy and with such amplitude as toreduce the gain of said amplifier, to obtain a resultant band-pass type input-output frequency response characteristic embracing the range between said cut-off frequencies and substantially suppressing frequencies outside said wave band to be transmitted. y

9. In an electric wave translation system, an electron valve ampiier having a normally broad frequency response characteristic, at least two paths for feeding back amplified output energy from a point of higher oscillation level to a point at a. lower oscillation level of said amplifier in inverse phase relation and with such amplitude as to reduce the gain of said amplifier, a low-pass network inserted inone of the feedback paths and a high-pass network inserted in the otherfeedback path, said networks having cut-off frequencies differing from each other by a predetermined amountl to cancel by negative feedback the response of said amplifier to frequencies below the cut-off frequency of vsaid low pass network and above the cut-off frequency of said high pass network, respectively.

'10. An electric system for translating a wave band of desired frequency range comprising an inputand an output and an amplifier connecting said input and output forming a main 'amplifying channel, the frequency response range of -said system substantially exceeding on both sides the frequency range of the wave -band toA be transmitted, a pair of high-pass and low-pass networks serially associated with said input circuit, the pass ranges of said networks overlapping each other vand a pair of feedback paths from said output to icV 6x said input circuit arranged to feed back amplified wave energy substantially in phase opposition to the incoming wave energy, one of said feedback paths including a high-pass 'network and the otherv feedback path including a low-pass network, the stop ranges of said latter networks overlapping each other to an extent substantially equalling said wave band so that thel cut-ofi' frequency of said latter high-pass network substantially coincides with the upper limit frequency and the cutoiI frequency of said latter low-pass network substantially coincides with the: lower limit frequency of said wave band, thereby obtaining a resultant band-pass type input-output-frequency range of the system embracing said wave band.

1l. An electric wave translation system com prising a pair of amplifiers having input and output circuits, feedback means associated with each amplifier for feeding back amplified energy to the input circuitsy in inverse phase relation to the input waves and with such amplitude as to reduce the gains of said amplifiers, means for impressing a common signal wave upon both input circuits of said amplifiers, further means for combining the outputs of said amplifiers, a first high-pass type selective network associated with the feed-back means of one amplifier, a second low-pass type selective network associated with the feedback means of the other amplifier, the cut-off frequencies of said networks differing from each other by a predetermined amount to obtain a resultant input-output band-pass type frequency response characteristic embracing the range be- -tween said cut-oft frequencies.

12. In an electric wave translation system, a first amplifier, a second amplifier, each of said amplifiers having an input and an output, feedback means associated with each amplifier for feeding back amplified wave energy in inverse re.- lation to the input energies and with such amplitude as to reduce the amplifying gain, means for impressing a common signal wave upon the inputs of both of said amplifiers, further means for combining the outputs of said amplifiers, a highpass filter connected in the feedback circuit of the first amplifier, a low-pass nlter connected in the feedback of the other amplifier, the cut-off frequencies of said filters differing from each other by a predetermined amount, a further highpass filter connected in the input of said first amplifier and having a cut-oil.' frequency equa? to the cut-of! frequency of said first low-pass filter, and a further low-pass filter connected in the input of said second amplier and having a cut-oi frequency equal to the cut-off frequency of said.

first high-pass filter.

13. An electric wave band translation system comprising an input and an output and an amplifler connecting said input and output, the frequency response range of said system substantially exceeding on both sides the frequency range of the wave band to be transmitted, a first Stop filter network associated with said input, and a feedback path from said output to said input arranged to feed back amplified wave energy substantially in phase opposition to the incoming wave energy and including a second stop filter network, said first network with its pass-range embracing said wave band and with its stoprange substantially covering said frequency response range outside one limit frequency of said wave band so that the cut-off frequency of said first network substantially coincides with said limit frequency, said second network with its stoprange embracing said wave band and substantially the stop range of said first network and with its pass-range substantially covering said frequency response range outside the other limit frequency of said band range so that the cut-oil.' frequency of said second network substantially coincides with said other limit frequency.

PAUL J, LARSEN. 

